When you create the record, when you update the record, or when you read it?
To do it when you create the record, you can set the default value of the
table's field to Date() (or Now() if you want both date and time).
To do it when you update a record, you must use a form. In the form's
BeforeUpdate event, put code to set the field to Date() (or Now())
To do it when you read the record, you'd need to put the logic in the form's
Current event.